The present invention relates to a resin bond grinding element having diamond particles distributed throughout and individually embedded in the resin bond. In particular, the present invention is directed to such a grinding element wherein a metallic coating is utilized on the diamond particles for purposes of improving the performance characteristics of the grinding element.
In order to determine the performance characteristics of the grinding element, it has been an accepted procedure for some period of time to utilize the grinding ratio. The grinding ratio is defined as the ratio of the volume of material removed from the workpiece during a given grinding operation to the volume of material worn away from the grinding element during that grinding operation.
For commercial purposes, grinding ratios are particularly significant in relation to so-called diamond grinding wheels, because as known throughout the trade, diamond grinding wheels are comparatively expensive. Therefore, to ascertain the commercial feasibility of using a diamond grinding wheel, one must know the performance characteristics of that grinding wheel, or specifically, the numerical value of the grinding ratio.
The numerical value of the grinding ratio, as defined above, is with respect to a given grinding operation. Thus for a particular operation, a given grinding wheel may have a given grinding ratio, whereas for another operation, that same grinding wheel may have a different grinding ratio. Even so, the grinding ratio for the particular operation to be performed is the significant factor to consider in determining cost.
Viewed consistent with the above, the present invention permits one to obtain a superior grinding ratio for a given grinding operation. In terms of superior grinding ratio, one must realize that this is a comparitive figure. Accordingly, for purposes of determining superiority, one can compare the grinding ratio which is obtained with a wheel made in accordance with the invention to, as a relative standard, the grinding ratio of the most efficient corresponding prior art grinding wheel, namely, a wheel which is otherwise identical to that made in accordance herewith except for the fact that the diamond grains are not coated with metal and the metal material of the coating is replaced by the material of the resin bond.
While the grinding ratio will vary with the particular grinding operation to be performed, one must not overlook the fact that the grinding ratio will also vary depending on the concentration of diamond grains within the grinding element.
Concentration is determined by taking the weight of diamond per unit volume of the finished grinding element. The grinding element for this purpose is the active section of the grinding wheel, namely, that section which contains diamond grains.
In the diamond grinding wheel art, a figure of 100 has been taken for many years as the concentration where the weight of the diamond grains is 72 carats/in.sup.3 or 4.4 carats/cm.sup.3 (carat = 0.2 grams). The carats per cubic inch or grams per cubic centimeter vary linearly with increases or decreases in concentration from such standard of 100 so that for a concentration of 50, for example, the diamond grains are present in an amount of 36 carats/in.sup.3 or 2.2 carats/cm.sup.3 and for a concentration of 200, the diamond grain is present in an amount of 144 carats/in.sup.3 or 8.8 carats/cm.sup.3.
Those skilled in the art have long recognized that for practical purposes, the concentration of diamond grinding wheels varies between approximately 25 and 200 (1.0 - 9.0 carats/cm.sup.3), but the preferable concentration is between 50 and 100 (2.2 - 4.4 carats/cm.sup.3) because the increase in grinding ratio with increased concentration for most operations tends to become insignificant at concentrations above 100 and because the wheel at least tends to glaze, rather than grind, when the ratio is less than 25.
Considering the aforesaid variables, it should be apparent that an evaluation of improved performance characteristics of any grinding element necessarily requires a comparison of grinding ratios of respective grinding elements as used for a particular grinding operation, where the respective elements have the same concentration. To phrase this another way, in order to determine improved performance of a diamond grinding element, one must use a "standard" grinding element for comparison purposes, which standard element has not only the same size and shape, but also the same diamond concentration as the element to be evaluated.
Since most all commercial grinding wheels have a grinding element or section with a diamond concentration between 50 and 100, and since within this range, the grinding ratio increases with an increase in concentration, realistic evaluations of performance characteristics desirably utilize wheels having grinding sections with concentrations at the upper end of the range, i.e., concentrations of 100.
In addition to understanding the importance of the meaning of grinding ratios and the importance of diamond concentration used in making the active grinding element of diamond grinding wheels, it is helpful to further understand the meaning of the accepted terminology in the art regarding particle size. In this regard, it should be stated at the outset that there is from a realistic standpoint no practical manner in which to make a diamond grinding wheel wherein the diamond particles all have the same size. To the contrary, in any grinding wheel having a diamond grinding section, diamond particles that are incorporated necessarily vary in size, although the sizes of particles in any diamond grinding element conventionally fall within a prescribed range.
When those skilled in the art speak in terms of a diamond grinding wheel, they are talking about diamond particles which have a particle size of a maximum of 420 microns (40 mesh). Obviously, then, the particles are so small that one does not distinguish between particle sizes by accurately measuring each particle as this would be a virtual impossibility.
In the circumstances, the diamond grinding element industry has developed standards relating to particle size. These standards effectively separate particle sizes into selected ranges. More particularly, particles having a size within a predetermined range are separated from a mass by sieving the mass through respective upper and lower screens wherein the upper screen has a larger mesh size than the lower screen. Particles which have a size larger than that within the selected range remain on the top screen and particles which have a size smaller than that within the selected range pass through the lower screen. Accordingly, in a random mass, one selectively screens particles having sizes between the upper limit prescribed by the upper screen mesh size and the lower limit prescribed by the lower screen mesh size.
Recognizing the above, and consistent with standards long established in the diamond grinding element industry, one finds that there are standard diamond sizes prescribed (at least as early as 1961) by ASTM Standard E11.
Consistent with this standard, the particle sizes are grouped according to screen mesh size and, in turn, micron particle size. This grouping is in accordance with the following chart:
Mesh Microns ______________________________________ 40-50 420-297 50-60 297-250 60-80 250-177 80-100 177-149 100-120 149-125 120-140 125-105 140-170 105-88 170-200 88-74 200-230 74-62 230-270 62-53 270-325 53-44 325-400 44-37 ______________________________________
Since the particles involved are so small and since one takes a random mass of particles from which particles having a size within the prescribed range are separated, the number of particles of any given size within the range varies such that there are a minimum number of particles having sizes at either end of the range and a maximum number of particles having sizes in the middle of the range so that the curve of distribution of particle sizes within the selected range is a Gauss distribution curve (generally bell shaped).
Notwithstanding the particle size distribution within any selected range, it should be apparent that the larger the selected range of particle sizes the coarser the grinding finish, and the smaller the selected range of particle sizes, the finer the grinding finish. Since the user intends to obtain a particular type of grinding finish with any diamond grinding wheel, particles in various different size ranges are not generally mixed within the same wheel. In fact, those familiar with the art recognize that if particles within a smaller size range are mixed with particles within a relatively larger size range, the particles within the relatively small size range are essentially ineffective because the work to be performed during the grinding operation falls on the particles in the larger size range.
Accordingly, in the diamond grinding wheel industry, it is to be expected that any given wheel incorporates diamond particles having a size within only one of the ranges set forth in the above chart.
Although commercial diamond grinding wheels can have particles within any one of the ranges shown in the chart, one of the most popular ranges for such wheels is the 100-120 mesh or 149-125 microns group. Therefore, for comparative analytical purposes, this middle group particle size range has been traditionally selected. At the same time, it is to be understood that the present invention is in no way limited to this particle size range, and instead, the same is mentioned here for explanatory purposes.